Implantable medical devices incorporating plasma polymerized film layers and charged amino acids

ABSTRACT

An implantable medical device, such as a stent, is disclosed comprising an amino acid or a polypeptide bonded to a plasma polymerized film layer formed on the device. A method of manufacturing the same is also disclosed.

CROSS REFERENCE

This application is a divisional of application Ser. No. 10/176,506, filed Jun. 21, 2002, now U.S. Pat. No. 7,056,523 which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of medical devices, such as stents. More particularly, this invention is directed to coatings which include chemically-bound polymers and/or oligomers of L-arginine.

2. Description of Related Art

In the field of medical technology, there is frequently a necessity to administer drugs locally. To provide an efficacious concentration to the treatment site, systemic administration of medication often produces adverse or toxic side effect for the patient. Local delivery is a preferred method in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Thus, local delivery produces fewer side effects and achieves more effective results.

A commonly employed technique for the local delivery of a drug is through the use of a medicated stent. One method of medicating a stent is by coating the stent with a polymer having a drug incorporated therein. L-arginine, or polypeptide oligomeric derivatives thereof, for example, those containing 5 to 20 amino acid units, is one example of a substance that can be delivered via a stent.

L-arginine is known to be a precursor of endothelium derived nitric oxide (NO). NO is synthesized from L-arginine, or its polymeric and/or oligomeric derivatives, by the enzyme NO synthase oxygenase, a homodimeric flavo-hemoprotein that catalyzes the 5-electron oxidation of L-arginine to produce NO and L-citrulline. Among other therapeutic properties, NO relaxes vascular smooth muscle cells and inhibits their proliferation. References describing beneficial therapeutic properties of L-arginine include U.S. Pat. No. 5,861,168 to Cooke et al. Cooke et al. teach that administering L-arginine, as the NO precursor, restores vascular NO activity in patients with endothelial vasodilator dysfunction due to restenosis. Moreover, Uemura et al. (Circulation, vol. 102, 2629-2635 (2000)), teach that the heptamer of L-arginine (R7) exhibits biological activity in the reduction of neointimial hyperplasia.

According to traditional techniques, L-arginine or its polymers and/or oligomers can be physically incorporated into a polymeric matrix for in situ local delivery. The embodiments of the present invention provide alternative methods for local delivery of L-arginine, or its polymers and/or oligomers, by an implantable device such as a stent.

SUMMARY

An implantable medical device is provided, comprising a plasma polymerized film layer and a polypeptide chemically bonded to the plasma polymerized film layer. The device can be, for example, a stent, such as a balloon expandable or self-expandable stent. The plasma polymerized film layer can be derived from an organic substance having carboxyl groups. The organic substance can include a low molecular weight organic acid such as acrylic acid, propionic acid, butyric acid, valeric acid, and methacrylic acid. Representative examples of the polypeptide can include poly(L-arginine), poly(D-arginine), poly(L-lysine), poly(D-lysine), poly(δ-guanidino-α-aminobutyric acid), or a racemic mixture of poly(L-arginine) or poly(D-arginine). In accordance with one embodiment, a surface of the device on which the plasma polymerized film layer is deposited includes a carbon deposit, wherein the plasma polymerized film layer is formed over and is bonded to the carbon deposit.

A method of forming a coating for an implantable medical device is also provided, comprising forming a plasma polymerized film layer on the device and bonding a polypeptide to the plasma polymerized film layer. In one embodiment, the method can additionally include implanting carbon deposits in the surface of the device prior to forming the plasma polymerized film layer, wherein the plasma polymerized film layer is formed on carbon deposits. The bonding of the polypeptide to the plasma polymerized film layer can be conducted by exposing the plasma polymerized film layer to a free base form of the polypeptide.

A stent comprising a plasma polymerized film layer and an amino acid ionically bonded to the plasma polymerized film layer is also provided.

A method of treatment of a patient having a vascular occlusion is also provided, comprising implanting a stent at the site of the occlusion, the stent including a plasma polymerized film layer having a first charge and administering to the patient a polypeptide having an opposing second charge wherein some of the polypeptide circulating through the blood stream of the patient will couple to the plasma polymerized film layer. The polypeptide can be administered orally, intravenously, or by a drug delivery catheter.

DETAILED DESCRIPTION

L-arginine, commonly abbreviated as “R” or “Arg,” also known as 2-amino-5-guanidinovaleric acid, has a formula NH═C(NH₂)—NH—(CH₂)₃—CH(NH₂)—COOH. L-arginine is an amino acid. Due to the presence of a strong basic guanidinium group, —NH—C(NH₂)═NH, carrying a partially uncompensated positive charge, L-arginine, its polymers and/or oligomers are highly cationic. For example, the heptamer of L-arginine has a pK_(a) of 13.2.

Polymers and/or oligomers of L-arginine that can be used are referred to herein as “PArg.” PArg are polycationic peptides comprising a plurality of repeating monomeric amino acid units and have a general formula H—[NH—CHX—CO]_(p)—OH, where “p” can be within a range of 5 and 1,000, typically, within a range of between 5 and 20. For example, a heptamer (designated R7) or a nonamer (R9), having p=7 and p=9, respectively, can be used. In the formula of PArg, “X” is 1-guanidinopropyl radical having the structure —CH₂—CH₂—CH₂—NH—C(NH₂)═NH. The terms “polymers and/or oligomers of L-arginine” and “PArg” are intended to include L-arginine in both its polymeric and oligomeric form.

In addition to PArg, other polycationic peptides can be alternatively used. Examples of such alternative polycationic peptides include poly(L-arginine), poly(D-arginine), poly(L-lysine), poly(D-lysine), poly(δ-guanidino-α-aminobutyric acid), racemic mixtures of poly(L-arginine) and poly(D-arginine), and chitosan.

The modification of the surface of the substrate, for example a surface of a stent, can include the following steps:

(a) implanting of carbon into the stent surface;

(b) functionalizing the stent surface containing the implanted carbon; and

(c) neutralizing the functionalized stent surface.

Prior to the implantation of carbon, the outer surface of the stent can be cleaned by, for example, argon plasma treatment or any other suitable cleaning method so as to remove contaminants and impurities that are introduced during the manufacturing process. To implant carbon into the surface, in accordance with one method, the stent can be treated with methane plasma in a Plasma Ion Implantation Unit (PIIU). PIIU is a system known to those having ordinary skill in the art. The methane plasma treatment can be conducted by enclosing the stent inside a stainless steel mesh followed by generating the methane plasma using a radio frequency power source. The length of time needed implant the necessary amount of carbon into the stent surface can be between about 2 to 5 minutes. Process parameters for the methane plasma that can be employed are listed in Table 1 below.

TABLE 1 The Parameters of the Methane Plasma for Implantation of Carbon into the Stent Surface (>99.9% by Volume of Methane) Parameter Parameter Range Exemplary Value Methane gas flow rate (sccm)  10 to 200 30 Volume of the PIIU chamber (cm³) — 2,000 Pressure (mTorr) 0.1 to 2.0 0.5 RF power (Watts)      10 to 1,000 100 RF frequency (MHz)       2 to 2,800 13.56 Bias voltage--stent (kV) −10 to −80 −50 Pulse width--stent (μs)   5 to 100 20 Frequency--stent (Hz)   DC to 2,000 200

Alternatively, the stent can be implanted with carbon by sputtering carbon from a carbon cage electrode. The stent can be placed inside the carbon cage electrode, which in turn can be put inside a plasma chamber. A plasma, for example, argon plasma, can be generated using a radio frequency power source, followed by applying a bias voltage to the carbon cage. In addition to argon, the gaseous medium can also include other components such as nitrogen, for example, 1:1 by volume blend of argon and nitrogen. The length of time needed implant the necessary amount of carbon into the stent surface can be between about 2 to 5 minutes. Process parameters for the argon plasma that can be employed are listed in Table 2. Under the described conditions, carbon can be sputtered from the carbon cage, accelerated towards the stent surface, and implanted into the stent surface.

TABLE 2 The Parameters of the Argon Plasma for Implantation of Carbon into the Stent Surface by Sputtering Carbon from a Carbon Electrode (>99.9% by Volume of Argon) Parameter Parameter Range Exemplary Value Gas flow rate (sccm)  10 to 500 50 Volume of the PIIU chamber (cm³) 2,000 Pressure (mTorr)  0.1 to 500   50 RF power (Watts)      10 to 1,000 200 RF frequency (MHz)       2 to 2,800 13.56 Bias voltage--stent (kV)  −5 to −30 −10 Bias voltage-carbon electrode (kV) 15 to 20 20 Pulse width--stent (μs)  5 to 20 20 Frequency--stent (Hz) DC-2,000 500 Bias voltage-grid of the PIIU (V)     −300 to −5,000 −1,000

Subsequent to the implantation of the carbon deposit, the stent can be cleaned using argon plasma. Process parameters for the argon plasma cleaning that can be employed are listed in Table 3.

TABLE 3 The Parameters of the Argon Plasma for Cleaning the Stent (>99.9% by Volume of Argon) Process Parameter Range Exemplary Value Argon flow rate (sccm)  10 to 250 250 Pressure (mTorr)  10 to 250 230 RF power (W)  50 to 450 400 RF frequency (MHz)   2 to 2800 13.54 Time (minutes)  3 to 30 5

After the stent surface is implanted with carbon and cleaned as described above, or by using any other acceptable method of carbon implantation and cleaning known to those having ordinary skill in the art, the stent surface can be functionalized by plasma polymerization as is understood by those having ordinary skill in the art. Regardless of which method is used for implanting carbon into the stent surface, carbon dioxide/acrylic acid plasma can be used for functionalization.

To functionalize, the carbon-implanted stent surface can be exposed to carbon dioxide/acrylic acid plasma to form an acrylate or acrylate-like polymer film layer on the surface of the stent. The carbon deposit on the surface of the stent provides a site at which covalent bonds can be formed with the plasma deposited acrylate or acrylate-like polymer film layer. One having ordinary skill in the art will recognize that some fragmentation of the acrylate can typically occur during the plasma polymerization process, resulting in an acrylate-like polymer layer of fragmented acrylate being formed on the carbon-implanted stent surface.

The plasma can be generated using a radio frequency power source under the conditions shown in Table 4. Instead of acrylic acid, those having ordinary skill in the art may select another low molecular weight saturated or unsaturated organic acid, for example, propionic acid, butyric acid, valeric acid, methacrylic acid, or mixtures thereof.

TABLE 4 The Conditions for Conducting Functionalization of the Stent Surface by Plasma Polymerization (CO₂/Acrylic Acid Plasma) Exemplary Parameter Parameter Range Value Acrylic acid gas rate (ml/min) 0.05-0.35 0.2 Carbon dioxide gas flow rate (sccm)  60-200 90 Power (W)  10-300 100 Volume of the PIIU chamber (cm³) — 2,000 Pressure (mTorr)  70 to 250 150 RF power (Watts)  50 to 250 100 RF frequency (MHz)      2 to 2,800 13.56

Carbon dioxide and acrylic acid can be pre-mixed by combining their respective streams flowing at rates shown in Table 4, and the combined stream can be fed into the PIIU chamber. Carbon dioxide can be supplied with acrylic acid to limit the rate of de-carboxylation which can occur with an organic acid in a plasma field. Acrylic acid can be vaporized prior to being combined with carbon dioxide. To vaporize acrylic acid, vacuum can be employed, for example, about 50 mTorr vacuum, and acrylic acid can be introduced by using, for example, a syringe pump.

In another embodiment, carbon dioxide and vaporized acrylic acid can be fed into the PIIU chamber as separate streams, at a flow rate for each respective stream as shown in Table 4.

The acrylic acid plasma can be applied for about 10 minutes, the time limit being dependent on the desired thickness of the acrylate or acrylate-like polymer film layer. The thickness of the plasma polymer film layer can be about 20 nm to about 500 nm, more narrowly about 70 nm to about 150 nm, for example, about 125 nm. In accordance with another embodiment, a pulsed plasma condition, as is understood by one of ordinary skilled in the art, can be employed for the deposition of polymer film layer. The process parameters are similar to those shown by Table 4, but for the power range being between about 60 W to about 450 W, for example, between about 250 W and about 350 W. For the implementation of pulsed plasma, the RF power can be pulsed at about 500 to 5,000 Hz, for example 1,000 Hz to about 1,250 Hz, using, for example, a square wave pulse sequence. The duty period, the time in which the power is on, can be between 15% and 100%, for example, 20% to 35%. With the use of pulsed plasma condition, the rate of de-carboxylation can be further limited.

Following deposition of the plasma-polymerized film layer, the plasma field can be purged with argon without an applied RF field to allow surface free radicals to be quenched by recombination prior to exposure to atmospheric oxygen. Table 5 provides parameters for this quenching process:

TABLE 5 The Conditions for Recombination of the Surface Free Radicals Process Parameter Range Exemplary Value argon — (>99.9% by volume) Gas flow rate (sccm)  30 to 300 230 Pressure (mTorr)  50 to 500 250 Time (minutes)  2 to 10 3

Following plasma polymerization, the carboxylated stent surface can be neutralized. To neutralize, the stent can treated with a diluted alkaline solution, for example, sodium hydroxide solution. The duration of treatment can be about 30 minutes. Following the process of neutralization, the stent can be washed with de-ionized water to remove the residual sodium hydroxide solution.

The neutralization procedure described above completes the process of modification of the stent surface. At this point, PArg, for example, R7 can be incorporated onto the stent by ionic coordination. To incorporate R7, the stent can be placed in a container containing an excessive amount of aqueous solution of R7, for example, the R7 solution in a free base form.

To obtain R7 in a free base form, R7 can be dissolved in water and a strong alkali can be added, such as potassium hydroxide KOH, raising the pH of the R7 solution to about 13. In a strongly alkaline environment, the guanidinium fragments of R7 are de-protonated and R7 in a free base form can be obtained as a result. In the free base form, R7 is a cation having the positive charge mainly concentrated on the imino nitrogen of the guanidinium group of R7 (NH=fragments). Such cation for the purposes of this invention is designated as R7⁺.

When the carboxylated stent surface containing neutralized polyanionic poly(acrylic acid)-like material is brought in contact with the aqueous solution of R7, the macromolecules of the poly(acrylic acid)-like material get solvated followed by charge separation, for example by dissociation, leading to the creation of carboxyl-anions as shown by reaction (I):

where X is a carbonized structure on the carbon-implanted stent surface to which poly(acrylic acid)-like material is attached.

R7⁺ cations are then ionically coordinated around negatively charged carboxyl-anions. Such coordination can be schematically shown by reaction (II):

Alternatively, R7 in a non-free base form can be also coordinated around the carboxyl-anions. Although the cationic nature of R7 in the non-free base form is not as pronounced as it is in the free base form, the positive charge on the guanidinium group is sufficient to cause coordination of R7 around the carboxylated stent surface.

When the modified neutralized stent is brought in contact with the R7 solution, the solution can be gently agitated, for example, by stirring for about 30 minutes, followed by rinsing the stent with de-ionized water and drying. Optionally, a polymeric topcoat can be then applied, examples of suitable polymers being poly(ethylene-co-vinyl alcohol) and poly(butyl methacrylate).

According to another embodiment, when R7 is brought in contact with the neutralized modified stent surface containing carboxyl groups, R7 can be trapped by being grafted to the carboxyl groups-containing stent surface by covalent conjugation. The carboxyl groups will react with amino groups of R7 to form an amide. One possible path of reaction can be illustrated by reaction (III) and the conditions of the reaction will be selected by those having ordinary skill in the art:

According to yet another embodiment, the stent, modified and neutralized as described above, can be placed at a diseased site in a blood vessel. Due to the polyanionic nature of the poly(acrylic acid) coating, the stent coating will carry a negative charge. R7 is then administered systemically, for example, intravenously, orally or through a perfusion balloon. R7 will be carried through the circulatory system and when R7 approaches the stent, some of positively charged R7 will coordinate around the negatively charged stent surface to form an ionic complex between R7⁺ and the poly(acrylic acid)-based anion, thus trapping R7. After untrapped R7 is cleared from the circulatory system, trapped R7 will still persist for some time providing an enhanced concentration of R7 at the vascular injury site.

The method of the present invention has been described in conjunction with a stent. The stent can be used in any part of the vascular system, including neurological, carotid, coronary, renal, aortic, iliac, femoral or any other peripheral vascular system. The stent can be balloon-expandable or self-expandable. There are no limitations on the size of the stent, its length, diameter, strut thickness or pattern.

The use of the coating is not limited to stents and the coating can also be used with a variety of other medical devices. Examples of the implantable medical device that can be used in conjunction with the embodiments of this invention include stent-grafts, grafts (e.g., aortic grafts), artificial heart valves, cerebrospinal fluid shunts, pacemaker electrodes, axius coronary shunts and endocardial leads (e.g., FINELINE and ENDOTAK, available from Guidant Corporation). The underlying structure of the device can be of virtually any design. The device can be made of a metallic material or an alloy such as, but not limited to, cobalt-chromium alloys (e.g., ELGILOY), stainless steel (316L), “MP35N,” “MP20N,” ELASTINITE (Nitinol), tantalum, tantalum-based alloys, nickel-titanium alloy, platinum, platinum-based alloys such as, e.g., platinum-iridium alloy, iridium, gold, magnesium, titanium, titanium-based alloys, zirconium-based alloys, or combinations thereof. Devices made from bioabsorbable or biostable polymers can also be used with the embodiments of the present invention.

“MP35N” and “MP20N” are trade names for alloys of cobalt, nickel, chromium and molybdenum available from Standard Press Steel Co. of Jenkintown, Pa. “MP35N” consists of 35% cobalt, 35% nickel, 20% chromium, and 10% molybdenum. “MP20N” consists of 50% cobalt, 20% nickel, 20% chromium, and 10% molybdenum.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications can be made without departing from this invention in its broader aspects. Therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention. 

1. A stent comprising a plasma polymerized film layer and a substance comprising a charged amino acid group, wherein the substance is attached to the plasma polymerized film layer, wherein the attachment comprises an ionic bond between the substance and the plasma polymerized film layer.
 2. The stent of claim 1, wherein the substance comprises L-arginine, D-arginine, L-lysine, D-lysine, δ-guanidino-α-aminobutyric acid, or combinations thereof.
 3. The stent of claim 1, wherein the amino acid group comprises a positive charge.
 4. The stent of claim 1, wherein the amino acid group comprises a negative charge.
 5. The stent of claim 1, wherein the substance comprises chitosan.
 6. The stent of claim 1, wherein the substance comprises a drug.
 7. The stent of claim 1, wherein the stent comprises a carbon deposit on the surface of the stent, and wherein the carbon deposit is attached to the plasma polymerized film layer.
 8. A stent comprising a plasma polymerized film layer and a substance comprising an amino acid, wherein the substance is chemically bonded to the plasma polymerized film layer, wherein the plasma polymerized film layer is derived from an organic substance having a carboxyl group.
 9. The stent of claim 8, wherein the organic substance comprises a low molecular weight organic acid or a combination of low molecular weight organic acids.
 10. The stent of claim 9, wherein the organic substance comprises a component selected from a group consisting of acrylic acid, propionic acid, butyric acid, valeric acid, methacrylic acid, and combinations thereof.
 11. The stent of claim 8, wherein the substance comprises L-arginine, D-arginine, L-lysine, D-lysine, δ-guanidino-α-aminobutyric acid, or a mixture thereof.
 12. The stent of claim 11, wherein the L-arginine is releasable for local delivery in a patient.
 13. The stent of claim 8, wherein a surface of the stent on which the plasma polymerized film layer is deposited includes a carbon deposit, wherein the plasma polymerized film layer is formed on and bonded to the carbon deposit. 